Epilepsy is a family of chronic neurological disorders characterized by recurring convulsive seizures, which result from abnormal, excessive neuronal activity in the central nervous system. It is estimated that about 65 million people worldwide have epilepsy. Epilepsy can arise from an imbalance in two major neurotransmitters that regulate brain neuronal activity, L-glutamate, an excitatory neurotransmitter, and γ-aminobutyric acid (GABA), an inhibitory neurotransmitter.
GABA is produced in GABAergic neurons from L-glutamate by the enzyme glutamic acid decarboxylase (GAD) (FIG. 1). GABA is then released into the synapse and transported to glial cells. The enzyme GABA aminotransferase (GABA-AT) in glial cells degrades GABA to succinic semialdehyde (SSA), which is further oxidized to succinate and enters the Krebs cycle. GABA-AT also converts α-ketoglutarate from the Krebs cycle to L-glutamate. Because there is no GAD in glial cells, this newly formed L-glutamate is not converted to GABA. It is instead converted to L-glutamine, which is then released from glial cells into the synapse and transported back to GABAergic neurons to complete the metabolic cycle of L-glutamate.
Low levels of GABA are linked to not only epilepsy, but also many other neurological disorders including Parkinson's disease, Alzheimer's disease, Huntington's chorea, and cocaine addiction. Raising GABA levels has proven effective in stopping recurring convulsive seizures in the treatment of epilepsy. However, GABA does not cross the blood-brain barrier (BBB); therefore, an increase in brain levels of GABA cannot be achieved by intravenous administration. Other possible routes to increased brain levels of GABA include enhancing the activity of GAD, the enzyme that makes GABA, inhibiting the activity of GABA-AT, the enzyme that degrades GABA, and inhibiting the reuptake of GABA by blocking the action of the GABA transporters.
One approach relates to the design of mechanism-based inactivators of GABA-AT; in particular, the design of unreactive compounds that require GABA-AT catalysis to convert them into a species that inactivates the enzyme. Because these molecules are not initially reactive, but require the catalytic activity of GABA-AT to become activated and form covalent bonds, indiscriminate reactions with off-target proteins, leading to undesired side effects, should be greatly reduced. Even at lower dosages, these inactivators can achieve the desired pharmacologic effects with enhanced potency and selectivity than conventional inhibitors.

Currently, the only FDA-approved inactivator of GABA-AT is the drug vigabatrin (2) (Scheme 1), which was first developed by Lippert et al., and is used for the treatment of epilepsy. However, a large dose of vigabatrin (˜1-3 g) needs to be taken daily, and there are many serious side effects that arise from its usage, including psychosis and permanent vision loss resulting from the damage of the retinal nerve fiber layer. As a result, the search for an alternative to vigabatrin in the treatment of epilepsy has been an ongoing concern in the art.
It has been determined that vigabatrin inactivates GABA-AT via two pathways: a Michael addition mechanism and an enamine mechanism, as shown in Scheme 1. In the Michael addition mechanism, the resulting Schiff base (4) from the reaction of vigabatrin and the lysine-bound PLP (1) on GABA-AT is subjected to γ-proton removal and tautomerization that leads to ketimine 5. An active-site nucleophile then reacts with Michael acceptor 5 to form 6, which is in equilibrium with 7. In the enamine mechanism, the Schiff base (4) is subjected to γ-proton removal and tautomerization through the vinyl bond, which leads to the release of enamine 10. Subsequent nucleophilic addition of 10 to the lysine-bound PLP on GABA-AT gives rise to 11.
The Michael addition mechanism and the enamine mechanism happen concurrently in a 70/30 ratio, respectively. It was discovered that ketimine 5 in the Michael addition mechanism, and enamine 10 in the enamine mechanism, underwent partial hydrolysis to form the α,β-unsaturated ketone (8) and the saturated ketone (12), respectively. While 8 is a reactive electrophile, possibly responsible for some side effects, 12 is not a reactive metabolite. From these findings, further study has been directed to vigabatrin analogs that either follow the enamine mechanism exclusively to avoid the formation of 8 or speed up the Michael addition pathway so that 5 would have much lower probability to undergo hydrolysis.
An energy minimized molecular model of vigabatrin bound to PLP in GABA-AT revealed that after tautomerization, the vinyl bond in 5 needs to rotate toward Lys-329 for the Michael addition to occur. Therefore, conformationally-restricted analogs such as 13 and 14 (FIG. 2) would prevent the rotation of the vinyl bond, thereby blocking the Michael addition mechanism. Experiments showed that 13 inactivated GABA-AT following the enamine mechanism exclusively. However, its potency remained low. In the alternative approach, conformationally-restricted analogs 15 and 16 have the vinyl bond readily pointed toward Lys-329 for rapid Michael addition to occur, thereby minimizing the hydrolysis of the ketimine intermediate. Experiments showed that 16 was 186 times more efficient in inactivating GABA-AT than vigabatrin. Furthermore, unlike vigabatrin, 16 did not inactivate or inhibit off-target enzymes, such as alanine aminotransferase and aspartate aminotransferase, and therefore is less likely to produce side effects. Indeed, 16 was tested in a multiple-hit rat model of infantile spasms, and the results showed that 16 suppressed spasms at doses of 0.1-1 mg/kg/day, which were >100-fold lower than those for vigabatrin. The spasms suppression by 16 stayed effective longer (3 days vs. 1 day for vigabatrin), and 16 also had a much larger margin of safety than vigabatrin.